inverter drive hydraulic pump

Hello all,

We have designed a 4 double acting driven system for a bio-medical research lab. They are using it for 2 years at fatigue testing of various samples. They now ask me whether we can lower the electricity costs. All of the cylinders have capacity of applying 3 mm peak to peak at 10 hertz in displacement controlled tests. However most of the tests require 0.2 to 0.5 mm peak to peak controlled tests and sometime they are using 2 3 pistons above 4. I though i may use an inverter or any recommended motor driver with the help of pressure transducer in the P line before direction valves. Or somewhere else you recommend maybe before main pressure relief valve. I think by controlling the motor revolutions per minute will both lower the electricity bill and also the thermal energy dissipated from the servo valves. Any help and recommendation would be appreciated.

What is the hydraulic power unit design? Is it only one pump? Is it a variable-displacement pump (swash plate), or a fixed-displacement pump with VFD, or a fixed-displacement pump with constant-speed motor and a relief valve?

If the hydraulic power unit is maintaining pressure with a relief valve, then you have the potential to save a lot of energy. A relief valve is very inefficient. The standard method of improving it is to use a swash-plate pump instead. That way, the motor will do less work one the system reaches pressure. I believe a fixed-displacement pump with a VFD is be somewhat more energy efficient, but the initial cost is more.

On a high-performance motion control system, it is very difficult to reduce the energy that is dropped across the valve. Reducing the system pressure may help, but there is a limit to how far it can be reduced and still maintain enough flow to perform the desired motion.

In recent years, research has figured out how to drive a cylinder directly from a hydraulic pump, without going through a servovalve, thereby saving large amounts of energy, but that works well for large human-controlled systems such as excavators, and not so well for systems that require precise, high-frequency control.

Waiting to hear more on what exactly your hydraulic power unit is.

Does the customer expect an increased frequency when the amplitude goes down? You have to always look at the maximum instantaneous flow required and make sure you can satisfy that through the pump (and if installed, the accumulator).

If the maximum instantaneous flow required from the pump/accumulator is less for the low amplitude tests, then the speed of the motor can be reduced.

Once you reduce the motor speed, you have to start looking at whether or not the fan on the motor can keep it cool at the slower speed. Sometimes a separate cooling fan on the motor which is run at constant speed is required.

In general with a VFD you are in a constant torque mode when operating below base frequency so theoretially you should be ok with pressure (pressure is directly proportional to torque). But you should look at the mechanical efficiency of the pump at the reduced speed to make sure it doesn’t drop off dramatically and keep you from making the required pressure. Most pump data sheets give information at standard speeds like 1500 and 1800 RPM. You may have to ask the pump maufacturer to get the efficiency numbers at different rotation speeds.

If you have multiple stage valves, be sure to account for pilot oil flow and pressure reuqirements as well.

If the conditions of the oil supplying the servo valves varries by any significant margins you will likely have to have different tuning parameters for the different conditions. This is pretty easy with the RMC and we use this quite often on a system we do where we vary the system pressure based on tonnage requred on a press for different parts.

Like Jacob said, the energy across the valve is not something that is easy to eliminate. I have seen designs with a servo motor, driving a fixed pump plumbed directly to a cylinder but I don’t think it would be easy at your frequencies. It definitely would be very expensive as you would need a servo motor, drive and pump for each cylinder!